Intravenous (IV) administration of tissue plasminogen activator (tPA) is an effective therapy for acute stroke when treatment is begun within 3 hours of symptom onset. The success of tPA therapy is attributed to the medication's ability to open occluded cerebral arteries before irreversible brain injury has occurred. Benefits of tPA, or any other stroke therapy, have not been clearly established for patients treated between 3-6 hours after stroke onset, a time period when a substantial number of patients present for evaluation. We believe this therapeutic failure has occurred because some patients treated 3-6 hours after symptom onset have already sustained severe, irreversible brain injury and others have undergone spontaneous recanalization of their occluded arteries. The results of our previous pilot trial, DEFUSE 1, indicate that baseline MRI profiles appear to differentiate patient subgroups that can benefit substantially from 3-6 hour IV tPA therapy from subgroups that do not benefit. DEFUSE 2 is an international, multicenter, open-label, pilot study of 100 patients who present 3-6 hours after stroke onset. Based on the MRI profiles identified in DEFUSE 1, patients will be treated with IV tPA. Two follow-up MRI scans will assess whether early reperfusion and/or recanalization occurs and establish the final infarct volume. The primary outcome assessment is based on the degree of neurologic improvement at 30 days. We hope to a) validate the results of the DEFUSE 1 study by confirming that specific MRI profiles are predictive of the clinical response to early reperfusion, and b) demonstrate that an automated analysis of DWI and PWI data can be performed quickly and accurately at multiple clinical sites utilizing MRI scanners from different manufacturers. PUBLIC HEALTH RELEVANCE: The key objective of DEFUSE 1 and 2 is to obtain sufficient preliminary data to optimally design a randomized trial that will assess the benefits of reperfusion therapy, administered 3-6 hrs after stroke symptom onset, using baseline MRI findings to select patents who are likely to have a robust clinical benefit from early reperfusion. We believe that the DEFUSE 2 study will clarify the key mediators of clinical outcomes in acute stroke patients and lead to a randomized trial that will demonstrate the efficacy of IV tPA therapy for a large population of patients who are currently ineligible for treatment. The key objective of DEFUSE 2 is to obtain sufficient preliminary data to optimally design a definitive study to assess the risks and benefits of treatment of stroke patients with a clot dissolving medication beyond the current 3 hour time window. We believe that the DEFUSE 2 study will clarify that specific MRI findings can identify patients who benefit from therapy 3-6 hours after symptom onset. These findings will eventually lead to effective therapies for a large population of stroke patients who are currently ineligible for treatment and substantially reduce stroke-related disability.